The invention relates to a shield construction for electrical devices which are placed near the antenna of mobile communication apparatus.
FIG. 8 shows a perspective view of a portable type mobile telecommunication radio apparatus. In FIG. 8, 1 is an enclosure of the radio apparatus. The enclosure and a cover of the apparatus are made of aluminum die casting. Electrical devices mounted inside of the enclosure are shielded from the radio frequency energy. 2 is a portable cover made of plastic and including a battery (not shown) inside thereof. 3 is a transmitting and receiving antenna which is mounted in the portable cover. 4 is a handset made of plastic and including a circuit board inside thereof.
FIG. 11 is a perspective viewe of the inside structure of a prior art handset 4. The handset 4 is assembled with an upper cover 41 and a lower cover 42. On the inside surfaces of the both covers, conductive coating paint is applied. Integrated circuit (IC) 44 and crystal oscillator 45 are mounted on a circuit board 40 which processes key signals and indication signals, and supplies signals to the IC 44 respectively. On the circuit board 40, there are many other circuit patterns which process on-off signal of the key board, but they are not shown in FIG. 11.
The operation of the above conventional art is explained hereinafter. The operator communicates using handset 4. In the mobile telecommunication radio apparatus, the transmitting frequency and the receiving frequency are used at the same time. If the antenna output is very large, or if the handset 4 reaches near the antenna 3, then noises are frequently emitted from the earpiece and speaker in the handset 4, or receiving sensitivity is sometimes suppressed.
Next, the phenomenon of the above noises and the receiving sensitivity suppression is explained hereinafter. IC 44 which includes a processor is mounted on the circuit board 40. The processor operates according to the clock signal generated in the crystal oscillator of IC 44. The clock signal is generally square waves and contains higher harmonic waves in it. Digital signals combined with high level signals and low level signals are also generated in the IC 44. The combined digital signals also contain many frequency component.
The conductive paint is applied on the inner sides of the upper cover 41 and lower cover 42 of the handset 4 so as to prevent interference from the radio waves. When upper cover 41 and lower cover 42 are combined, the circuit board 40 is surrounded by the conductive shield construction. Therefore the radio waves transmitted from the antenna 3 are attenuated by the shield construction before reaching the circuit board 40. When the upper cover 41 and lower cover 42 are combined however, the inside of the handset 4 can not be thoroughly shielded because of the deflection of a portion of the covers 41,42 when they are assembled together.
As stated above the coverage of the shield construction is usually incomplete. In the region of the keypad, shield construction is also incomplete. The shielding effect at the keyboard region decreases because the conductive paint is applied only in the lattice shape around the keys. Therefore if the transmitting power from the antenna 3 is very large, or if the handset 4 reaches near the antenna, the signal level of the transmitting wave which reaches the circuit board 40 becomes relatively large. If the circuit pattern distance from the key contact point to the IC 44 is long, the transmitting wave comes through the circuit board and reaches to IC 44.
As described above, the signals in the IC 44 contain clock signals and digital signals including higher harmonics. When transmitting waves above a certain level reach the IC 44, the frequency of the transmitting wave and the frequency of the clock signal or the digital signal including higher harmonics are mixed in the non-portions of the semiconductors in the IC 44. As a result, some frequency component the same as the receiving frequency are generated in the mobile telecommunication apparatus.
If the receiving signal level is equal to or smaller than the frequency components of the same frequency generated in the IC 44, receiving sensitivity deteriorates. Also, audio frequency components generated in the IC 44 are demodulated in the radio apparatus 1. As a result, receiving sensitivity is disturbed in a weak electric field area and noises are emitted out from the earpiece. Also, if the transmitting signal level becomes larger, it is rectified by the semiconductors in the IC 44. As a result, the rectified voltage changes the bias level and causes malfunction of the apparatus or breaks down the circuit elements.